1. Field of the Invention
The invention pertains to a slave cylinder for a hydraulic system between an internal combustion engine and a transmission of a motor vehicle for actuating a friction clutch, the slave cylinder comprising a cylinder housing, which is concentric to a transmission shaft and in which a ring-shaped piston is held, the piston being connected to a clutch-release bearing for hydraulic actuation of the friction clutch, where the cylinder housing is supported on a wall of the transmission to absorb the actuating forces acting on the clutch-release bearing.
2. Description of the Related Art
A clutch-release device for motor vehicles with hydraulic actuation is known from German Patent Application DE 198 19 970.8. The slave cylinder of this device actuates a clutch-release unit, which is concentric to a transmission axis and can engage and disengage the clutch of a motor vehicle. The clutch-release unit consists of a housing and a piston, which is moved by a pressurized medium; the medium is controlled by a control ring with a pressure line connection and a vent hole. As a result of the cooperation of the control ring with a control groove in the piston, the effect is produced that the slave cylinder always travels the same distance relative to that traveled by the pedal throughout the life of the clutch, even after the clutch has suffered wear.
In a discussion of the previously mentioned clutch-release device, the wear compensation realized over the entire service life of the unit is of less interest than the means used to attach the housing to the wall of the transmission. For this purpose the housing has a flange, which must be screwed to the transmission wall. It can be seen in FIG. 2 that the piston in the housing is connected to a clutch-release bearing, which, as a result of the precise fit of the piston in the housing, has no freedom to move in the radial direction. The clutch-release bearing is in almost continuous contact with the tongues of the diaphragm spring of the clutch, and the tongues almost always have axial run-out at the point where they come in contact with the clutch-release bearing. The tongues thus cause the clutch-release bearing and the piston of the slave cylinder to wobble in correspondence with the first harmonic of the engine rpm's. As a result, increased wear can occur both at the contact points between the clutch-release bearing and the tongues of the diaphragm spring and in the slave cylinder. The wobbling movements are transmitted not only during the clutch-engaging operation but also when the clutch is not released. Whereas the diaphragm springs and thus the tongues are deformed during the release operation and the axial run-out is at least partially compensated, the full axial run-out is present at the clutch-release bearing while the clutch is in the unreleased state. Finally, manufacturing inaccuracies, specifically center offsets and eccentricities attributable in particular to the heat treatment of the diaphragm springs, also allow relative movements to occur between the clutch-release bearing and the tongues.